1. Field of the Invention
The present invention relates to a wireless local access network (WLAN), and more particularly, to a channel access mechanism in a very high throughput (VHT) WLAN system and a station supporting the channel access mechanism.
2. Discussion of the Related Art
With the advancement of information communication technologies, various wireless communication technologies have recently been developed. Among the wireless communication technologies, a wireless local access network (WLAN) is a technology whereby super high-speed Internet access is possible in a wireless fashion in homes or businesses or in a region providing a specific service by using a portable terminal such as a personal digital assistant (PDA), a laptop computer, a portable multimedia player (PMP), etc.
Ever since the institute of electrical and electronics engineers (IEEE) 802, i.e., a standardization organization for WLAN technologies, was established in February 1980, many standardization works have been conducted. In the initial WLAN technology, a frequency of 2.4 GHz was used according to the IEEE 802.11 to support a data rate of 1 to 2 Mbps by using frequency hopping, spread spectrum, infrared ray communication, etc. Recently, the WLAN technology can support a data rate of up to 54 Mbps by using orthogonal frequency division multiplex (OFDM). In addition, the IEEE 802.11 is developing or commercializing standards of various technologies such as quality of service (QoS) improvement, access point (AP) protocol compatibility, security enhancement, radio resource measurement, wireless access in vehicular environments, fast roaming, mesh networks, inter-working with external networks, wireless network management, etc.
In the IEEE 802.11, the IEEE 802.11b supports a data rate of up to 11 Mbps by using a frequency band of 2.4 GHz. The IEEE 802.11a commercialized after the IEEE 802.11b uses a frequency band of 5 GHz instead of the frequency band of 2.4 GHz and thus significantly reduces influence of interference in comparison with the very congested frequency band of 2.4 GHz. In addition, the IEEE 802.11a has improved the data rate to up to 54 Mbps by using the OFDM technology. Disadvantageously, however, the IEEE 802.11a has a shorter communication distance than the IEEE 802.11b. Similarly to the IEEE 802.11b, the IEEE 802.11g realizes the data rate of up to 54 Mbps by using the frequency band of 2.4 GHz. Due to its backward compatibility, the IEEE 802.11g is drawing attention, and is advantageous over the IEEE 802.11a in terms of the communication distance.
The IEEE 802.11n is a technical standard relatively recently introduced to overcome a limited data rate which has been considered as a drawback in the WLAN. The IEEE 802.11n is devised to increase network speed and reliability and to extend an operational distance of a wireless network. More specifically, the IEEE 802.11n supports a high throughput (HT), i.e., a data processing speed of up to 540 Mbps at a frequency band of 5 GHz, and is based on a multiple input and multiple output (MIMO) technique which uses multiple antennas in both a transmitter and a receiver to minimize a transmission error and to optimize a data rate. In addition, this standard may use a coding scheme which transmits several duplicated copies to increase data reliability and also may use the OFDM to support a higher data rate.
Meanwhile, a basic access mechanism of an IEEE 802.11 medium access mechanism (MAC) is a carrier sense multiple access with collision avoidance (CSMA/CA) combined with binary exponential backoff. The CSMA/CA mechanism is also referred to as a distributed coordinate function (DCF) of the IEEE 802.11 MAC, and basically employs a “listen before talk” access mechanism. In this type of access mechanism, a station (STA) listens a wireless channel or medium before starting transmission. As a result of listening, if it is sensed that the medium is not in use, a listening STA starts its transmission. Otherwise, if it is sensed that the medium is in use, the STA does not start its transmission but enters a delay duration determined by the binary exponential backoff algorithm.
The CSMA/CA mechanism also includes virtual carrier sensing in addition to physical carrier sensing in which the STA directly listens the medium. The virtual carrier sensing is designed to compensate for a limitation in the physical carrier sensing such as a hidden node problem. For the virtual carrier sending, the IEEE 802.11 MAC uses a network allocation vector (NAV). The NAV is a value transmitted by an STA, currently using the medium or having a right to use the medium, to anther STA to indicate a remaining time before the medium returns to an available state. Therefore, a value set to the NAV corresponds to a duration reserved for the use of the medium by an STA transmitting a corresponding frame.
One of procedures for setting the NAV is a exchange procedure of a request to send (RTS) frame and a clear to send (CTS) frame. The RTS frame and the CTS frame include information capable of delaying transmission of frames from receiving STAs by reporting upcoming frame transmission to the receiving STAs. The information may be included in a duration filed of the RTS frame and the CTS frame. After performing the exchange of the RTS frame and the CTS frame, a source STA transmits a to-be-transmitted frame to a destination STA.
FIG. 1 is a diagram showing an IEEE 802.11 MAC architecture including a DCF. Referring to FIG. 1, a service of the DCF is used to provide a point coordination function (PCF) and a hybrid coordination function (HCF). The HCF includes an enhanced distributed channel access (EDCA) and an HCF controller channel access (HCCP). The HCF does not exist in an STA not supporting quality of service (QoS). On the other hand, both the DCF and the HCF exist in an STA supporting QoS. The PCF is an arbitrary function in all STAs. Details of the DCF, PCF, EDCA, and HCCF are disclosed in section 9 of the “MAC sublayer function description” in the IEEE 802.11-REVma/D9.0 October 2006 standard, and thus descriptions thereof will be omitted herein. The contents of the above standard are incorporated herein by reference.
With the widespread use of WLAN and the diversification of applications using the WLAN, there is a recent demand for a new WLAN system to support a higher throughput than a data processing speed supported by the IEEE 802.11n. However, an IEEE 802.11n medium access control (MAC)/physical layer (PHY) protocol is not effective to provide a throughput of 1 Gbps or more. This is because the IEEE 802.11n MAC/PHY protocol is designed for an operation of a single STA, that is, an STA having one network interface card (NIC), and thus when a frame throughput is increased while maintaining the conventional IEEE 802.11n MAC/PHY protocol, a resultant additional overhead is also increased. Consequently, there is a limitation in increasing a throughput of a wireless communication network while maintaining the conventional IEEE 802.11n MAC/PHY protocol, that is, a single STA architecture.
Therefore, to achieve a data processing speed of 1 Gbps or more in the wireless communication system, a new system different from the conventional IEEE 802.11n MAC/PHY protocol (i.e., single STA architecture) is required. A very high throughput (VHT) system is a next version of the IEEE 802.11n WLAN system, and is one of IEEE 802.11 WLAN systems which have recently been proposed to support a data processing speed of 1 Gbps or more in a MAC service access point (SAP). The VHT system is named arbitrarily. To provide a throughput of 1 Gbps or more, a feasibility test is currently being conducted for the VHT system using 4×4 MIMO and a channel bandwidth of 80 MHz.
Meanwhile, a data processing speed of 1 Gbps or more, which is set as a target throughput in a VHT system, denotes an aggregate throughput. On the other hand, a target throughput in one-to-one communication between STAs is at least 500 Mbps in the VHT system. This implies that performance or an offered load of an STA supporting VHT (hereinafter, simply referred to as a ‘VHT STA’) may not exceed 500 Mbps. In a case where the offered load of the VHT STA is less than 1 Gbps (e.g., 500 Mbps), the target throughput of the VHT system cannot be achieved when one VHT STA is allowed to use an entire channel similarly to the conventional channel access method.
In addition, there is a problem in that efficiency is not high in the aforementioned CSMA/CA channel access method used in the IEEE 802.11 WLAN. For example, a data processing speed in a MAC SAP is only 50 to 60% of a data processing speed in a PHY SAP. Therefore, in order to achieve a data processing speed of 1 Gbps or more in the MAC SAP of the VHT system, the data processing speed of the PHY SAP needs to be about 1.5 to 2 times higher than 1 Gbps. However, the conventional IEEE 802.11n PHY technique has difficulty in providing such a processing speed.